As a hammer drill, for example, an apparatus such as disclosed in Patent Document 1 is known. In this apparatus, a tool spindle (tool holder) having a bit-installable front end is rotatably supported in a housing, and a motor is disposed in the housing with an output shaft thereof oriented in a direction perpendicular to an axis of the tool holder. Further, in the housing, an impact mechanism is provided which comprises a piston configured to make a reciprocating motion by means of a crank mechanism actuated according to a rotation of the output shaft and an impactor configured to move in synchronization with the piston by the action of an air spring. With this configuration, upon activation of the motor, the piston is caused to reciprocate by means of the crank mechanism, and the impactor synchronized with the piston strikes a bit directly or indirectly through an interjacent element such as an impact bolt.
Meanwhile, a shaft (speed reduction shaft) provided with a bevel gear and being in mesh with a bevel gear on the tool holder side is rotatably supported in a position parallel to the output shaft between the tool holder and the output shaft of the motor, two gears meshed with another shaft provided closer to the motor are rotatably fitted onto the shaft, and a connecting slider (clutch pin) is provided at a shaft center of the shaft so as to be slidable along an axial direction of the shaft and projections thereof are configured to be engaged with or disengaged from the gear and the shaft in accordance with sliding positions of the connecting slider, so that a clutch mechanism configured to permit switching of operation modes is provided. To be more specific, when the projections of the connecting slider are engaged with the gear, the gear operates integrally with the shaft to transmit a rotary motion of the output shaft to the shaft, so that the tool holder is rotated through the bevel gear. This mode of operation is called as a hammer drill mode. On the contrary, when the projections of the connecting slider are engaged with the housing side, the transmission of the rotary motion from the gear to the shaft is interrupted so that a striking motion only is generated, and a rotary motion of the shaft is restricted to lock the rotation of the tool holder. This mode of operation is called as a hammer mode.
In a state where the projections of the connecting slider are free from engagement with both the gear and the housing, the transmission of the rotary motion from the gear to the shaft is interrupted so that a striking motion only is generated, and the shaft is allowed to rotate freely so that the tool holder becomes freely rotatable. This mode of operation is called as a neutral mode.    Patent Document 1: Japanese Laid-open Patent Publication No. 2002-28878